Normal Modes in Hexagonal Boron Nitride

Pyrolytic boron nitride, a rather anisotropic material among the III-V compounds, has been investigated in reflection and transmission, using polarized infrared radiation in the spectral range from 100 ${\mathrm{cm}}^{\ensuremath{-}1}$ to 3000 ${\mathrm{cm}}^{\ensuremath{-}1}$. The reflection spectra have been analyzed by means of a Kramers-Kronig analysis and a classical oscillator fit. The results indicate one infrared active lattice mode for in-plane motion of the particles and one for out-of-plane motion, in accordance with the boron-nitride structure reported by Pease and the resulting group-theoretical selection rules. In addition, one Raman line has been found corresponding to a lattice mode for in-plane motion. Furthermore, the absorption spectrum exhibits several minor peaks which are probably due to 2-phonon combination processes.